Social relationships influence diverse aspects of human health, from mood to mortality. Furthermore, many psychiatric disorders list psychopathologies of social behavior as one of their core symptoms (eg. autism, social anxiety disorder, antisocial personality disorder). Prairie voles are socially monogamous rodents that provide a means of studying the neural mechanisms that underlie attachment. While decades of research have now examined the basis of monogamy, the mechanisms underlying specific affiliation to non-mate peers remain largely unknown?despite the frequency and importance of peer relationships. In this project we examine peer relationships in prairie voles and contrast them to mate relationships, as well as to peer relationships in a related non-monogamous species. We specifically focus on the role of reward and dopamine transmission because mate relationships are highly rewarding and require activation of dopamine (type D2) receptors in order to form. Dopamine (type D1) receptors are involved in the maintenance of the pair bond and rejection of other potential social contacts. Evidence from a related vole?the meadow vole? suggests that peer affiliation is not dopamine dependent in this non-monogamous species, and the role of reward has not been tested. Three specific aims investigate the role of reward and dopamine across relationship types. Aim 1 will determine whether social reward is an important feature of peer relationships using monogamous prairie voles and non-monogamous meadow voles. Voles will be tested for conditioned place preferences associated with social contact, and reward value will be quantified in voles trained to lever press for different social and non- social incentives. Aim 2 will use similar methods to assess the role of reward in mate versus peer relationships in prairie voles. Aim 3 will assess the role of dopamine in peer affiliation by measuring the effects of dopamine receptor manipulations on behavior, and by measuring the effects of social manipulations on the densities of dopamine (D1 and D2) receptors. These studies will provide basic information on the role of reward and dopamine in peer relationships in a monogamous mammal, laying groundwork for better understanding of peer relationships in humans. The contrasts between a) prairie vole vs. meadow vole peer relationships, and b) prairie vole peer vs. mate relationships will determine whether these mechanisms are more consistent across types of relationships within a species, or within peer relationships across species. This will yield important insights into how these findings can be appropriately translated to human health, and which research models will provide them.